"It is an ancient adversary and an old enemy with whom we wage our battle: six thousand years are now nearly completed since the devil first attacked man....the divine arrangement containing seven thousand of years" Cyprian (Treatise 11; Preface, 2; On the Exhortation to Martyrdom, 11).
"And 6,000 years must needs be accomplished, in order that the Sabbath may come, the rest, the holy day 'on which God rested from all His works.' For the Sabbath is the type and emblem of the future kingdom of the saints, when they 'shall reign with Christ,' when He comes from heaven, as John says in his Apocalypse: for 'a day with the Lord is as a thousand years.'" - Hippolytus (On Daniel, 2:4)